kconfig: remove redundant IEEE 802.15.4 defaults or selections

All IEEE 802.15.4 drivers are now automatically enabled if they are
'okay' in DT and all of its dependencies are 'y', including
CONFIG_IEEE802154. This means individual driver enablement is not
necessary anymore in Kconfig.defconfig files or samples/tests. Boards
need to still make sure any dependencies are enabled, e.g. SPI bus in
some cases.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>